Tuesday, April 03, 2018

California's Century of Growth

A Century of Growth is an interactive cartogram visualizing population growth in California in the one hundred years from 1900-2000. In fact the visualization includes six different cartograms which show the population in each Californian county every twenty years from 1900 to 2000.

This Leaflet based cartogram shows the population growth in California by changing the size of each county proportional to the county's population. If you change the year on the map the counties change size to show the population in each county in the selected year. When you select a year on the map the map sidebar also updates to provide a brief explanation of the population changes in California over the previous twenty years.

A Century of Growth also includes a good definition of what a cartogram actually is. Jake has used a semi-contiguous cartogram in his visualization of California's growing population. The USGS Cartogram Types provides good definitions of contiguous, non-contiguous and Dorling cartograms. This image of the three different types of cartogram visualizing the population of California is also a neat illustration of the difference between contiguous and non-contiguous cartograms.

In a non-contiguous cartogram the enumeration units (in this case California's counties) do not have to maintain connectivity with each other. The counties can grow or shrink in size and keep their geographical shape. In a non-contiguous cartogram the units keep their connections. This leads to the bubbling effect seen above, where the geographical shape of the counties become distorted.

Jake has used a semi-contiguous cartogram. Because some large counties have small populations while other geographically smaller counties have much larger populations some of the connectivity between the different counties is lost. The loss in geographical shape is mitigated by including a small geographically correct inset map. The colors of the counties on the cartogram and the inset map correspond to help the map user read the distorted cartogram. The user can also hover over counties on the map to view its name.